After Chapter 7, in the street around the Arena, there will be two children talking; a brother and a sister. After Chapter 19, return to the same place and you'll only find the devoted brother. After talking to him return to Ariela's Tavern. When you exit again the devoted brother will have stolen the Castle's Pass, so you'll have to chase him throughout the city.
Sometimes he'll stop to rest; in that moment you'll have to enter Seek Mode to locate him. If you loose him, just listen to the people's conversations to get clues about where he is. Sometimes he'll crash into other people, so take advantange of that and get to him.
The first time you trap him he'll escape, but the second time the chase will end. You'll appear back in the Tavern. There you'll talk with him and Ariela, and you'll get the Pass back. You'll recieve an Agility +20 bonus and you will also find a Dragon Fang on the Tavern's lower floor.
After the talk you'll return to the street of the Arena, where the boy will explain his sister's illness. After completing this side quest, Chapter 21 will become available. You can do the side quest to access the chapter before the story takes you there.